Lake Aydat Research Articles

Interdisciplinary strategy to assess the impact of meteorological variables on the biochemical composition of the rain and the dynamics of a small eutrophic lake under rain forcing

Abstract. We present an interdisciplinary investigation of the links between the macro and microphysical properties of rain, the biochemical composition of rain, meteorological parameters, and their impacts on the phytoplankton dynamics of a mountain lake. In order to document this interdisciplinary scientific question, Lake Aydat in the French Massif Central mountains was fitted with a set of high-resolution atmospheric radars, a disdrometer, and a precipitation collector in 2020. In parallel, the lake was monitored via sensors and water sampling. To illustrate the potential of this novel experimental setup, we present a case study of a rain event that occurred in September 2020 and during which, three contrasting sub-periods were identified based on the type of rain (convective or stratiform). Using our high-temporal resolution monitoring, we show that the origin of the air mass mainly influences the chemical composition of the rain, which depends on the rain type. In contrast, the photosynthetic cell concentration in the rain is mainly influenced by meteorological variables, predominantly below-cloud scavenging. The very low concentrations of photosynthetic cells found in rain events cannot directly impact the lake's phytoplankton abundance. In contrast, the rain rate directly impacted the lake's thermal stratification during the convective event. The response of the phytoplankton depends on the genus and, interestingly, three cyanobacteria, Microcystis, Coelomoron, and Merismopedia, showed a systematic response to rain events with a sudden decrease in abundance at the lake surface immediately after rain events. In contrast, the abundance of green algae (Elakatothrix), picocyanobacteria (Synechocystis and Synechococcus), and diatoms (Asterionella and Melosira) gradually increased following the rain events, but with a lower intensity compared to the cyanobacteria species. These different phytoplankton responses to the same rainfall event could play a key role in phytoplankton dynamics in the temperate zone. Our results highlight the importance of high-frequency and time resolution monitoring of both atmosphere and lake to better understand the adaptive strategies of cyanobacteria following rain events.

Single Cell Analysis Reveals a New Microsporidia-Host Association in a Freshwater Lake.

Microsporidia are a large group of obligate intracellular eukaryotic parasites. Recent studies suggest that their diversity can be huge in freshwater lake ecosystems especially in the < 150-µm size fraction. However, little is known about their hosts and therefore their impact on the trophic food web functioning. In this study, single cell analysis and fluorescence microscopy were used to detect new host-parasite association within rotifer communities in lake Aydat (France). Our analysis showed the existence of a potential new species belonging to the Crispospora genus able of infecting the rotifer Kellicottia with a high prevalence (42.5%) suggesting that Microsporidia could have a great impact on the rotifer populations' regulation in lakes.

Anthropogenic versus natural control on lacustrine sediment yield records from the French Massif Central

A quantitative assessment of historical sediment yields (SY) was performed using sediment budgets from lacustrine records located in the Mont Dore and Cezallier volcanic provinces (French Massif Central). A source-to-sink approach combining hydro-acoustic images, organic geochemistry (Rock-Eval and quantitative organic petrography) and radiocarbon dating of sediment cores has been adopted on three lake-catchment systems, namely Pavin, Chauvet and Montcineyre. SY was estimated from the quantification of red Amorphous Particles (rAP), a terrigenous organic tracer identified in both soils and sediments. Historical SY range between 3 and 320 t km−2.yr−1, which is comparable to the magnitude reached in other European lake-based SY records in similar geographical and climatic settings. Comparison of recent SY with predicted values of soil erosion rates from the RUSLE2015 model highlights large differences linked to scale differences between the model at plot scale and lake-based SY reflecting erosion export from the catchment to the lake. In this sense, the role of peatlands as sediment traps within two studied catchments must be considered to explain the large differences between modelled soil erosion rates and reconstructed SY data. SY differences between sites can be firstly attributed to morphology, size and lithology of the catchments as well as to vegetation cover whereas fluctuations reconstructed for each record seem to be mainly related to human-induced land use management. Historical SY from Chauvet and Montcineyre synchronously recorded two events in 850 CE and 1450 CE, respectively. The first marked the rise of SY to their maxima following land-use changes in the catchments. Nearby palaeoenvironmental records from Lake Aydat, Chambedaze peatland, and Espinasse marsh suggest this rise was consecutive to intensification of agro-pastoral activities recorded at regional scale. The second event followed a land-use shift characterized by a ten to fifteen-fold decrease in SY values. The driver remains unclear but could be possibly related to historical events causing a demographic decline (i.e., the Black Plague and/or the Hundred Years War) and/or cultural adaption in response to the onset of the Little Ice Age. Overall, both records suggest erosion in the area has been historically more susceptible to human-induced land use change rather than to precipitation and temperature changes induced by climate variability of the past millennium.

6700 years of diatom changes related to land use and climatic fluctuations in the Lake Aydat catchment (Auvergne, France): Coupling with cyanobacteria akinetes, pollen and non-pollen palynomorphs data

Nowadays Lake Aydat is an important site for tourism activities but blooms of cyanobacteria induced some restriction for aquatic activities by humans and their livestock. Thus, it was important to understand the lake's history as it should help in the development of restoration strategies to improve water quality in the future. Lake Aydat had a complex history affected by climatic changes and human influences. Two different sedimentary periods of deposition were observed (6700 ± 200–3180 ± 90, and 1770 ± 60 cal. BP – nowadays) separated by a mass wasting deposit. The lower unit was dominated by Stephanodiscus diatom species (S. medius, S. minutulus and S. parvus) and two Aulacoseira species (A. pusilla and A. subarctica) and the upper layer by Aulacoseira subarctica and its form recta along with Lindavia radiosa and a series of “Fragilariod” taxa. The eutrophic diatom species increase, associated with the lower layer, was most likely related to prehistorical human activities within the watershed (progressive change from a wooded landscape to an agricultural patchy landscape). The Middle Bronze Age degradation was noted as a period with a rise in planktonic and tychoplanktonic diatom species. The lower diatom zone in the upper unit marked the final transition to a domination of agricultural practices and high human pressure and a whole new set of environmental factors affecting the lake since 1750 cal. BP. Increased nutrient enrichment was observed and the Lindavia radiosa presence in the lake was associated with the beginning of hemp (Cannabis sativa) cultivation and an increase in agricultural pressure. High concentrations of cyanobacteria akinetes were associated with the final transition and the maximum of hemp cultivation, when the lake became very eutrophic. For future restoration strategies, improving the water quality will be a challenge as high concentrations of akinetes and nutrients are observed since the Antiquity in the lake.

Akinetes and ancient DNA reveal toxic cyanobacterial recurrences and their potential for resurrection in a 6700-year-old core from a eutrophic lake

In order to evaluate the recurrence of toxic cyanobacterial blooms and to determine the survival capabilities of the resistance cells through time, a sedimentary core spanning 6700 years was drilled in the eutrophic Lake Aydat. A multiproxy approach (density, magnetic susceptibility, XRF, pollen and non-pollen palynomorph analyses), was used initially to determine the sedimentation model and the land uses around the lake. Comparison with the akinete count revealed that Nostocales cyanobacteria have been present in Lake Aydat over a six thousand year period. This long-term cyanobacterial recurrence also highlights the past presence of both the anaC and mcyB genes, involved in anatoxin-a and microcystin biosynthesis, respectively, throughout the core. The first appearance of cyanobacteria seems to be linked to the natural damming of the river, while the large increase in akinete density around 1800 cal.yr BP can be correlated with the intensification of human activities (woodland clearance, crop planting, grazing, etc.) in the catchment area of the lake, and marks the beginning of a long period of eutrophication. This first investigation into the viability and germination potential of cyanobacteria over thousands of years reveals the ability of intact akinetes to undergo cell divisions even after 1800 years of sedimentation, which is 10 times longer than previously observed. This exceptional cellular resistance, coupled with the long-term eutrophic conditions of this lake, could partly explain the past and current recurrences of cyanobacterial proliferations.

Taxonomical Resolution and Distribution of Bacterioplankton Along the Vertical Gradient Reveals Pronounced Spatiotemporal Patterns in Contrasted Temperate Freshwater Lakes.

We examined the relationship between viruses and co-occurring bacterial communities across spatiotemporal scale in two contrasting freshwater lakes, namely meromictic Lake Pavin and dimictic Lake Aydat (Central France). Next-generation sequencing of 16S rRNA genes suggested distinct patterns in bacterioplankton community composition (BCC) between the lakes over depths and seasons. BCC were generally dominated by members of Actinobacteria, Proteobacteria, and Bacteroidetes covering about 95% of all sequences. Oxygen depletion at the bottom waters in Aydat and existence of permanent anoxia in the monimolimnion of Pavin resulted in the occurrence and dominance of lesser known members of lake communities such as Methylotenera, Methylobacter, Gallionella, Sulfurimonas, and Syntrophus in Pavin and Methylotenera and Sulfuritalea in Aydat. Differences in BCC appeared strongly related to dissolved oxygen concentration, temperature, viral infection, and virus-to-bacteria ratio. UniFrac analysis indicated a clear distinction in BCC when the percentage of viral infected bacterial cells and virus-to-bacteria ratio exceeded a threshold level of 10% and 5, respectively, suggesting a link between viruses and their potential bacterial host communities. Our study revealed that in both the lakes, the prevailing environmental factors across time and space structured and influenced the adaptation of bacterial communities to specific ecological niches.

Akinetes May Be Representative of Past Nostocalean Blooms: a Case Study of Their Benthic Spatiotemporal Distribution and Potential for Germination in a Eutrophic Lake.

Monitoring of water and surface sediment in a French eutrophic lake (Lake Aydat) was carried out over a 2-year period in order to determine whether akinetes in sediment could be representative of the most recent bloom and to estimate their germination potential. Sediment analysis revealed two akinete species, Dolichospermum macrosporum and Dolichospermumflos-aquae, present in the same proportions as observed for the pelagic populations. Moreover, similar spatial patterns observed for vegetative cells in the water column and akinete distributions in the sediment suggest that akinetes in the sediment may be representative of the previous bloom. However, the relationship between akinetes in the sediment and vegetative cells in the water column was not linear, and other factors may interfere. For example, our results highlighted horizontal transport of akinetes during the winter. The benthic overwinter phase did not seem to influence the percentages of intact akinetes, which remained stable at approximately 7% and 60% for D. macrosporum and D. flos-aquae, respectively. These percentages may thus be the result of processes that occurred in the water column. The intact overwintering akinetes showed germination rates of up to 90% after 72 h for D. flos-aquae or 144 h for D. macrosporum The difference in akinete germination rates between these two species demonstrates different ecological strategies, which serve to expand the window for germination in time and space and thus optimize colonization of the water column by nostocalean cyanobacteria.IMPORTANCE Cyanobacteria have the ability to proliferate and to form blooms. These blooms can then affect the local ecology, health, and economy. The akinete, a resistant cell type that persists in sediment, is an important intermediate phase between previous and future blooms. We monitored the water column and the surface sediment of a French eutrophic lake (Lake Aydat) to investigate the relationship between vegetative cells in the water column and akinetes in the sediment. This study focused on the characterization of spatiotemporal akinete distributions, cellular integrity, and germination potential. Species-specific ecological strategies were highlighted and may partly explain the temporal succession of species in the water column. Akinetes may also be used to understand past nostocalean blooms and to predict future ones.

Benthic Archives Reveal Recurrence and Dominance of Toxigenic Cyanobacteria in a Eutrophic Lake over the Last 220 Years.

Akinetes are resistant cells which have the ability to persist in sediment for several decades. We have investigated the temporal distribution of akinetes of two species, Dolichospermum macrosporum and Dolichospermum flos-aquae, in a sediment core sampled in Lake Aydat (France), which covers 220 years. The upper part, from 1907 to 2016, the number of akinetes fluctuated but stayed at high concentrations, especially for D. macrosporum in surface sediment (with the maximal value close to 6.105 akinetes g DW−1 of sediment), suggesting a recurrence of blooms of this species which was probably closely related to anthropic eutrophication since the 1960s. Before 1907, the abundance of akinetes of both species was very low, suggesting only a modest presence of these cyanobacteria. In addition, the percentage of intact akinetes was different for each species, suggesting different ecological processes in the water column. This percentage also decreased with depth, revealing a reduction in germination potential over time. In addition, biosynthetic genes of anatoxin-a (anaC) and microcystin (mcyA) were detected. First results show a high occurrence of mcyA all down the core. In contrast, anaC gene was mostly detected in the surface sediment (since the 1980s), revealing a potentially more recent occurrence of this cyanotoxin in Lake Aydat which may be associated with the recurrence of blooms of D. macrosporum and thus with anthropic activities.

Characterization of akinetes from cyanobacterial strains and lake sediment: A study of their resistance and toxic potential

Nostocalean cyanobacteria are known to proliferate abundantly in eutrophic aquatic ecosystems, and to produce several cyanotoxins, including anatoxin-a. In this study, we investigated both the resistance and toxic potential of the akinetes (resistant cells), using cyanobacterial cultures and akinetes extracted from the sediment of Lake Aydat (France) sampled in the winter and spring. Intact and lysed akinetes were differentiated using a double control based on the autofluorescence of akinetes and SYTOX-green staining. The percentage of resistant akinetes found in several different abiotic stress conditions was highly variable, depending on the species and also on the sampling season. Thus, the resistance of akinetes and their ability to germinate seems to follow a species-specific process, and akinetes can undergo physiologic changes during the sedimentary phase of the Nostocale life cycle. This study also revealed the first evidence of anatoxin-a genes in akinetes, with anaC and anaF genes detected in akinetes from all cyanobacterial producer cultures. The low number of anaC genes, almost exclusively detected using nested PCR, in the sediment at Lake Aydat suggests a limited but existent past population of toxic Nostocales in this lake. Given the key role of akinetes in the annual cycle and subsequent summer proliferation, it can be interesting to integrate the surveillance of akinetes in the management of lakes exposed to recurrent cyanobacterial blooms.

Co-occurrence of microcystin and anatoxin-a in the freshwater lake Aydat (France): Analytical and molecular approaches during a three-year survey

Cyanobacterial mass occurrence is becoming a growing concern worldwide. They notably pose a threat to water users when cyanotoxins are produced. The aim of this study was to evaluate the occurrence and the dynamics of two cyanotoxins: microcystin (MC) and anatoxin-a (ANTX-a), and of two of the genes responsible for their production (respectively mcyA and anaC) during three consecutive bloom periods (2011, 2012 and 2013) in Lake Aydat (Auvergne, France). MC was detected at all sampling dates, but its concentration showed strong inter- and intra-annual variations. MC content did not correlate with cyanobacterial abundance, nor with any genera taken individually, but it significantly correlated with mcyA gene abundance (R2=0.51; p=0.042). MC content and mcyA gene abundance were maximal when cyanobacterial abundance was low, either at the onset of the bloom or during a trough of biomass. The LC-MS/MS analysis showed the presence of ANTX-a in the 2011 samples. To our knowledge, this is the first report of the presence of this neurotoxin in a French lake. The presence of ANTX-a corresponded to the only year for which Anabaena did not dominate the cyanobacterial community alone, and several cyanobacterial genera were present, including notably Aphanizomenon. anaC gene detection by PCR was not coherent with ANTX-a presence, both gene and toxin were never found for a same sample. This implies that molecular tools to study genes responsible for the production of anatoxin-a are still imperfect and the development of new primers is needed. This study also highlights the need for better monitoring practices that would not necessarily focus only on the peak of cyanobacterial abundance and that would take cyanotoxins other than MC into account.

Tracking long-term human impacts on landscape, vegetal biodiversity and water quality in the Lake Aydat catchment (Auvergne, France) using pollen, non-pollen palynomorphs and diatom assemblages

Palaeoenvironmental studies allow the assessment of long-term human–climate–environmental interactions, and furnish valuable tools for the sustainable management of lacustrine ecosystems. A good example is the multi-proxy study of Lake Aydat's 19 m sedimentary core. Previous research revealed the role of climate and human activities on lake sedimentation, and identified two sedimentary units (6700 ± 200 to 3180 ± 90 and 1770 ± 60 cal. yr BP to present) separated by an erosive mass-wasting deposit (Lavrieux et al., 2013a). Pollen, non-pollen palynomorphs (e.g. fungal and algal spores, rotifer resting eggs), and diatom-based trophic reconstructions have been used to track the impacts of past land use on landscape evolution, vegetal biodiversity and water quality. Palaeoenvironmental data were also compared with local archaeo-historical datasets which allowed refined landscape reconstructions, especially for late Antiquity. The results obtained demonstrate that even Neolithic and Bronze Age human activities (between ca. 4600 and 4300 cal. yr BP and between ca. 3900 and 3500 cal. yr BP) had a discernible influence on catchment vegetation and lacustrine trophic dynamics of Lake Aydat, underlining the vulnerability of the ecosystem. Recurrent and complex models of past vegetation changes, phases of water nutrient over-enrichment and lake resilience were identified and related to grazing activities, but also to land use practises, which have been overlooked in Auvergne, such as mountain agriculture and hemp retting.

Parasitic chytrids sustain zooplankton growth during inedible algal bloom.

This study assesses the quantitative impact of parasitic chytrids on the planktonic food web of two contrasting freshwater lakes during different algal bloom situations. Carbon-based food web models were used to investigate the effects of chytrids during the spring diatom bloom in Lake Pavin (oligo-mesotrophic) and the autumn cyanobacteria bloom in Lake Aydat (eutrophic). Linear inverse modeling was employed to estimate undetermined flows in both lakes. The Monte Carlo Markov chain linear inverse modeling procedure provided estimates of the ranges of model-derived fluxes. Model results confirm recent theories on the impact of parasites on food web function through grazers and recyclers. During blooms of “inedible” algae (unexploited by planktonic herbivores), the epidemic growth of chytrids channeled 19–20% of the primary production in both lakes through the production of grazer exploitable zoospores. The parasitic throughput represented 50% and 57% of the zooplankton diet, respectively, in the oligo-mesotrophic and in the eutrophic lakes. Parasites also affected ecological network properties such as longer carbon path lengths and loop strength, and contributed to increase the stability of the aquatic food web, notably in the oligo-mesotrophic Lake Pavin.

6700 yr sedimentary record of climatic and anthropogenic signals in Lake Aydat (French Massif Central)

A 19 m long sedimentary record retrieved in Lake Aydat (French Massif Central) covers the last 6700 yr at a high resolution. A multiproxy approach (density, magnetic susceptibility, XRF, Rock-Eval, pollen and non-pollen palynomorph analyses and a historical archives study) was used to characterise and propose a model of sedimentation. The high deposition rate results from the combined effects of the high suspension load of the river, autochthonous production and the narrow shape of the incised fluvial valley dammed by a lava flow c. 8550 years ago. Two contrasted periods (6700±200 to 3180±90 cal. BP, and 1770±60 cal. BP to present) were characterized. The lower unit (mid Holocene) displays a fine and regular lamination and holds a single, major, flood deposit. This unit is capped by an erosive mass-wasting deposit triggered c. 1770±60 cal. BP. The upper unit (late Holocene) is made of organic-rich and fine-grained faintly laminated sediment, with numerous interbedded flood deposits and diatom blooms. The sedimentation was principally controlled by climatic forcings until c. 1100 cal. BP, accompanied by detrital events linked to human activities around the lake. Then, a more detrital input attested by numerous and recurrent flood deposits can be linked to the intensification of a persistent anthropogenic impact on the catchment. Two phases of lake eutrophication are highlighted: 1200–1130 cal. BP, as a result of increased anthropogenic pressure, and the current phase that could have started c. 150 cal. BP.

Methoxy-serratenes in a soil under conifers and their potential use as biomarkers of Pinaceae

In the frame of a study aimed at defining new soil molecular biomarkers that could attest to former land use and that could also be screened in lake sediments for palaeoenvironmental reconstruction, we analysed the 2–4cm depth layer of a soil developed under a conifer forest in the Lake Aydat catchment (French Massif Central). The lipid fraction of the sample contained 12 serratenoids bearing at least a methoxyl (OMe) group at C-3 or C-21 and various additional functional groups (alcohol, ketone or acetate). A survey of the literature provided indubitable evidence that these compounds are typical for Pinaceae species, in agreement with the surrounding vegetation at the sampling site. Due to the economic and ecological importance of the taxa, the presence of these highly specific compounds in soil, sediments or peat could therefore help in unravelling the timing of forestry activity.

Free fatty acids in Lake Aydat catchment soils (French Massif Central): sources, distributions and potential use as sediment biomarkers

PurposeEighteen soils were sampled in the Lake Aydat catchment in order to analyse free fatty acid (FA) content; FAs are considered to be among the most amenable biomarkers to mobilisation by runoff waters. The majority of the study area has soil cover consisting of grasslands or forest since the 2nd World War, although some covers having changed more recently.Material and methodsThe soil studied all developed on volcanic rocks (andisols). The bulk organic matter (OM) content of the samples was characterized by Rock-Eval (RE) pyrolysis. The FAs were determined by gas chromatography–mass spectrometry (GC-MS) analysis of isolated and derivatized (methylation and trimethylsilylation) FA fractions.Results and discussionFew low molecular weight compounds (LMW; i.e., <C20) were detected; FA distributions were dominated by even numbered-carbon high molecular weight (HMW; ≥C20) normal FAs and difunctionalized FAs that included: dicarboxylic acids (diFAs), n-alkylcarboxylic acids (nFAs), and α- and ω-hydroxycarboxylic acids (αHOFAs and ωHOFAs). The distributions and abundances of HMW terms of all families (which can be all considered as representative of terrestrial OM source) displayed only slight differences. These differences were rationalized by the following ratios: (C26:0 + C28:0)/ΣCeven nFAs, C22/C24 di-FAs, and C20−/C20+ω-HOFAs). Soils from areas that had changed use recently consistently displayed intermediate ratio values typical of their double inheritance. All grassland soils and some samples from intermediate areas contained notable amounts of the bile acid deoxycholic acid that testifies to their present or recent use for cattle breeding.ConclusionsDespite the variety and the abundance of all HMW FAs in soils, work done previously on Lake Aydat sediments found only nFAs (Stefanova, M. and Disnar, J. R. 2000. Composition and early diagenesis of FAs in lacustrine sediments, Lake Aydat (France). Org Geochem 31, 41–55). These results suggest we should question the importance of the watershed contribution, the source (plant or soil) and mode of transportation of the FAs to the lake sediments.

Phytoplankton chytridiomycosis: community structure and infectivity of fungal parasites in aquatic ecosystems

Fungal parasitism is recurrent in plankton communities, especially in the form of parasitic chytrids. However, few attempts have been made to study the community structure and activity of parasites at the natural community level. To analyse the dynamics of zoosporic fungal parasites (i.e. chytrids) of phytoplankton, samples were collected from February to December 2007 in two freshwater lakes. Infective chytrids were omnipresent in lakes, with higher diversity of parasites and infected phytoplankton than in previous studies. The abundance and biomass of parasites were significantly higher in the productive Lake Aydat than in the oligomesotrophic Lake Pavin, while the infection prevalence in both lakes were similar and averaged about 20%. The host species composition and their size appeared as critical for chytrid infectivity, the larger hosts being more vulnerable, including pennate diatoms and desmids in both lakes. The highest prevalence (98%) was noted for the autumn bloom of the cyanobacterium Anabaena flosaquae facing the parasite Rhizosiphon crassum in Lake Aydat. Because parasites killed their hosts, this implies that cyanobacterial blooms, and other large size inedible phytoplankton blooms as well, may not totally represent trophic bottlenecks because their zoosporic parasites can release dissolved substrates for microbial processes through host destruction, and provide energetic particles as zoospores for grazers. Overall, we conclude that the parasitism by zoosporic fungi represents an important ecological driving force in the food web dynamics of aquatic ecosystems, and infer general empirical models on chytrid seasonality and trophodynamics in lakes.

High Lytic Infection Rates but Low Abundances of Prokaryote Viruses in a Humic Lake (Vassivière, Massif Central, France)

We explored the abundance and infection rates of viruses on a time series scale in the euphotic zone of the humic mesotrophic Lake Vassivière (Massif Central, France) and compared them to nonhumic lakes of contrasting trophy (i.e., the oligomesotrophic Lake Pavin and the eutrophic Lake Aydat) located in the same geographical region and sampled during the same period. In Lake Vassivière, the abundances of virus-like particles (range, 1.7 × 10(10) to 2.6 × 10(10) liter(-1)) were significantly (P < 0.001) lower than in Lakes Pavin and Aydat. The percentage of virus-infected prokaryotic cells (mean, 18.0%) was significantly higher (P < 0.001) in Vassivière than in Pavin (mean, 11.5%) and Aydat (mean, 9.7%). In Vassivière, the abundance of prokaryotes was a good predictor (r = 0.78, P < 0.001) of the number of virus-like particles, while the potential grazing rate from heterotrophic nanoflagellates was positively correlated to the viral infection rate (r = 0.75, P < 0.001; n = 20), indicating the prevalence of cycling interactions among viruses, prokaryotes, and grazers, which is in agreement with past experiments. The absence of correlation between chlorophyll a concentrations (Chl) and viral parameters suggested that the resources for the lytic activity of viruses in Vassivière were mainly under allochthonous control, through host activity. Indeed, compilation of data obtained from several nonhumic lakes in the French Massif Central revealed that Chl was positively correlated to the abundance of virus-like particles at concentrations above 0.5 μg Chl liter(-1) and negatively at concentrations below 0.5 μg Chl liter(-1), suggesting that phytoplankton-derived resources could force prokaryotic growth to attain a certain threshold level when the host availability is sufficient to boost the proliferation of viruses. Therefore, based on the high level of lytic infection rates in Lake Vassivière, we conclude that viruses are key agents for prokaryotic mortality and could influence the food web dynamics in humic lakes, which may ultimately depend on the internal cycling of resources and, perhaps, mainly on the allochthonous inputs and the associated humic substances.

Exploring and quantifying fungal diversity in freshwater lake ecosystems using rDNA cloning/sequencing and SSU tag pyrosequencing

Water samples were collected along transects from the shore to the centre of two French lakes: the deep, volcanic, oligomesotrophic and low allochthonic-impacted Lake Pavin, and the productive and higher allochthonic-impacted Lake Aydat. The biodiversity was analysed using two approaches: the classical approach consisting of cloning/sequencing of the 18S, ITS1, 5.8S, ITS2 and partial 28S region using primers designed for fungus sequences, and the pyrosequencing of 18S rRNA hypervariable V2, V3 and V5 regions using two primer sets (one universal for eukaryotes and one for fungi). The classical approach yielded 146 (Lake Pavin) and 143 (Lake Aydat) sequences, corresponding to 46 and 63 operational taxonomic units (OTUs) respectively. Fungi represented half of the OTUs identified in Lake Pavin and 30% in Lake Aydat, and were dominated by sequences from Chytridiomycota found throughout Lake Pavin but mostly in the central pelagic zone of Lake Aydat. The pyrosequencing approach yielded 42,064 (Pavin) and 61,371 (Aydat) reads, of which 12-15% and 9-19% reads were assigned to fungi in Lakes Pavin and Aydat respectively. Chytridiomycota members were also dominant among these reads, with OTUs displaying up to > 33-fold overrepresentation in the centre compared with the riparian areas of Lake Aydat. Besides fungi, both approaches revealed other major eukaryote groups, with the highest diversity in the central areas of lakes. One of the major findings of our study was that the two lakes displayed contrasting spatial distributions, homogenous for Lake Pavin and heterogeneous for Lake Aydat, which may be related to their peculiarities. This study represents the first unveiling of microbial eukaryote and fungus diversity assessed with two complementary molecular methods, and is considered a major milestone towards understanding the dynamics and ecology of fungi in freshwater lake ecosystems, which are directly link to the abundance and distribution of taxa.

Biogeochemical Modelling of a Seasonally Anoxic Lake: Calibration of Successive and Competitive Pathways and Processes in Lake Aydat, France

A reactive transport model was developed to describe seasonal variations of biogeochemical and physical processes in Lake Aydat. The model includes physical processes such as vertical mixing, sedimentation and advection related to inflows into the lake and biogeochemical conversion processes in the water column and in the sediment surface layer. The reactions described in the model include primary redox reactions such as primary production, aerobic and anaerobic respiration, methanogenesis and secondary reactions established between oxidants and reducers produced by the primary reactions. After adjusting various kinetic constants, the model reasonably reproduced the main features of seasonal variations of dissolved oxygen and nitrate depth profiles and pH. The reactive transport model was also used to quantify the relative importance of different biogeochemical pathways. For instance, ferrous denitrification seems to play an important role when stratification is increasing.

Use of Calcofluor White for Detection, Identification, and Quantification of Phytoplanktonic Fungal Parasites

We propose a routine protocol based on size fractionation of pelagic samples and the use of the fluorochrome calcofluor white (which binds to beta-1,3 and beta-1,4 polysaccharides) for diagnosing, identifying, and counting chitinaceous fungal parasites (i.e., the sporangia of chytrids) of phytoplankton. The protocol was applied to freshwater samples collected during different seasons (spring and summer/autumn) in two lakes whose trophic statuses varied. Because few samples were collected (i.e., two dates per site), the findings are considered preliminary and mainly a "proof of concept" rather than a valid comparison of sites versus seasons. The results from the proposed protocol indicate higher diversity of infected host and parasite communities than in previous studies. Chytrid epidemics were omnipresent, infecting diverse phytoplankton host communities, primarily diatoms, chlorophytes, and colonial and filamentous cyanobacteria. The diversity and numerical abundance of sporangia and of hosts, and the prevalence of infection (range, <1 to 24% of total host cells) as well, increased from the oligotrophic Lake Pavin to the eutrophic Lake Aydat, while the temporal changes in parasites were apparently more influenced by the host community composition. We conclude that the proposed protocol offers a valid method for the quantitative ecology of chytrid epidemics in aquatic ecosystems and food web dynamics.